So is killing them.
The trick is dealing with the armor.
For that you can either use something to shred the armor (grenades, hell cannon, deceptor, serker shred armor skill), use melee or bash (a very effective technique is jet jump a heavy with Brawler and high strength, bash the Siren [damage from bash is weight of weapon * strength, so using a hel cannon to bash with 20 str and Brawler does 150 dmg] and if she isn’t dazed or there are other enemies around, cast war cry.
If you shred the armor, they go down very easily with an AR (can be as little as 2-3 bursts) or shotgun (no more than 2 shots).
Also, a Siren MCing a unit or even two is not the end of the world.
I covered them in my guide, but I didn’t update to include the nerfs to Sirens from last patch.
Edit:
And that’s all you need, especially for the first Sirens that you meet. The problem with Sirens, as with many other things in the game, is that PP does a poor job of explaining its fairly complex mechanics. And by the time you learn them, it decides to “reward” you with Terminators that don’t require any tactical thought, but that’s another story…